relibc_openlibm/slatec/d9chu.f

*DECK D9CHU
      DOUBLE PRECISION FUNCTION D9CHU (A, B, Z)
C***BEGIN PROLOGUE  D9CHU
C***SUBSIDIARY
C***PURPOSE  Evaluate for large Z  Z**A * U(A,B,Z) where U is the
C            logarithmic confluent hypergeometric function.
C***LIBRARY   SLATEC (FNLIB)
C***CATEGORY  C11
C***TYPE      DOUBLE PRECISION (R9CHU-S, D9CHU-D)
C***KEYWORDS  FNLIB, LOGARITHMIC CONFLUENT HYPERGEOMETRIC FUNCTION,
C             SPECIAL FUNCTIONS
C***AUTHOR  Fullerton, W., (LANL)
C***DESCRIPTION
C
C Evaluate for large Z  Z**A * U(A,B,Z)  where U is the logarithmic
C confluent hypergeometric function.  A rational approximation due to Y.
C L. Luke is used.  When U is not in the asymptotic region, i.e., when A
C or B is large compared with Z, considerable significance loss occurs.
C A warning is provided when the computed result is less than half
C precision.
C
C***REFERENCES  (NONE)
C***ROUTINES CALLED  D1MACH, XERMSG
C***REVISION HISTORY  (YYMMDD)
C   770801  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   890531  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
C   900720  Routine changed from user-callable to subsidiary.  (WRB)
C***END PROLOGUE  D9CHU
      DOUBLE PRECISION A, B, Z, AA(4), BB(4), AB, ANBN, BP, CT1, CT2,
     1  CT3, C2, D1Z, EPS, G1, G2, G3, SAB, SQEPS, X2I1,  D1MACH
      LOGICAL FIRST
      SAVE EPS, SQEPS, FIRST
      DATA FIRST /.TRUE./
C***FIRST EXECUTABLE STATEMENT  D9CHU
      IF (FIRST) THEN
         EPS = 4.0D0*D1MACH(4)
         SQEPS = SQRT(D1MACH(4))
      ENDIF
      FIRST = .FALSE.
C
      BP = 1.0D0 + A - B
      AB = A*BP
      CT2 = 2.0D0 * (Z - AB)
      SAB = A + BP
C
      BB(1) = 1.0D0
      AA(1) = 1.0D0
C
      CT3 = SAB + 1.0D0 + AB
      BB(2) = 1.0D0 + 2.0D0*Z/CT3
      AA(2) = 1.0D0 + CT2/CT3
C
      ANBN = CT3 + SAB + 3.0D0
      CT1 = 1.0D0 + 2.0D0*Z/ANBN
      BB(3) = 1.0D0 + 6.0D0*CT1*Z/CT3
      AA(3) = 1.0D0 + 6.0D0*AB/ANBN + 3.0D0*CT1*CT2/CT3
C
      DO 30 I=4,300
        X2I1 = 2*I - 3
        CT1 = X2I1/(X2I1-2.0D0)
        ANBN = ANBN + X2I1 + SAB
        CT2 = (X2I1 - 1.0D0)/ANBN
        C2 = X2I1*CT2 - 1.0D0
        D1Z = X2I1*2.0D0*Z/ANBN
C
        CT3 = SAB*CT2
        G1 = D1Z + CT1*(C2+CT3)
        G2 = D1Z - C2
        G3 = CT1*(1.0D0 - CT3 - 2.0D0*CT2)
C
        BB(4) = G1*BB(3) + G2*BB(2) + G3*BB(1)
        AA(4) = G1*AA(3) + G2*AA(2) + G3*AA(1)
        IF (ABS(AA(4)*BB(1)-AA(1)*BB(4)).LT.EPS*ABS(BB(4)*BB(1)))
     1    GO TO 40
C
C IF OVERFLOWS OR UNDERFLOWS PROVE TO BE A PROBLEM, THE STATEMENTS
C BELOW COULD BE ALTERED TO INCORPORATE A DYNAMICALLY ADJUSTED SCALE
C FACTOR.
C
        DO 20 J=1,3
          AA(J) = AA(J+1)
          BB(J) = BB(J+1)
 20     CONTINUE
 30   CONTINUE
      CALL XERMSG ('SLATEC', 'D9CHU', 'NO CONVERGENCE IN 300 TERMS', 2,
     +   2)
C
 40   D9CHU = AA(4)/BB(4)
C
      IF (D9CHU .LT. SQEPS .OR. D9CHU .GT. 1.0D0/SQEPS) CALL XERMSG
     +   ('SLATEC', 'D9CHU', 'ANSWER LT HALF PRECISION', 2, 1)
C
      RETURN
      END